use exact terser version

Author: guybedford

package.json

@@ -19,7 +19,7 @@
     "rollup": "^0.58.2",
     "source-map": "^0.5.3",
     "systemjs": "^0.19.46",
-    "terser": "^3.8.1",
+    "terser": "3.8.1",
     "traceur": "0.0.105"
   },
   "devDependencies": {